Manual Section... (3) - page: db

NAME

SYNOPSIS

DESCRIPTION

dbopen()
is the library interface to database files.
The supported file formats are btree, hashed and UNIX file oriented.
The btree format is a representation of a sorted, balanced tree structure.
The hashed format is an extensible, dynamic hashing scheme.
The flat-file format is a byte stream file with fixed or variable length
records.
The formats and file format specific information are described in detail
in their respective manual pages
btree(3),
hash(3)
and
recno(3).

dbopen()
opens
file
for reading and/or writing.
Files never intended to be preserved on disk may be created by setting
the
file
argument to NULL.

The
flags
and
mode
arguments are as specified to the
open(2)
routine, however, only the
O_CREAT,
O_EXCL,
O_EXLOCK,
O_NONBLOCK,
O_RDONLY,
O_RDWR,
O_SHLOCK,
and
O_TRUNC
flags are meaningful.
(Note, opening a database file
O_WRONLY
is not possible.)

The
type
argument is of type
DBTYPE
(as defined in the
<db.h>
include file) and
may be set to
DB_BTREE,
DB_HASH,
or
DB_RECNO.

The
openinfo
argument is a pointer to an access method specific structure described
in the access method's manual page.
If
openinfo
is NULL, each access method will use defaults appropriate for the system
and the access method.

dbopen()
returns a pointer to a
DB
structure on success and NULL on error.
The
DB
structure is defined in the
<db.h>
include file, and contains at
least the following fields:

These elements describe a database type and a set of functions performing
various actions.
These functions take a pointer to a structure as returned by
dbopen(),
and sometimes one or more pointers to key/data structures and a flag value.

type

The type of the underlying access method (and file format).

close

A pointer to a routine to flush any cached information to disk, free any
allocated resources, and close the underlying file(s).
Since key/data pairs may be cached in memory, failing to sync the file
with a
close
or
sync
function may result in inconsistent or lost information.
close
routines return -1 on error (setting
errno)
and 0 on success.

del

A pointer to a routine to remove key/data pairs from the database.

The argument
flag
may be set to the following value:

R_CURSOR

Delete the record referenced by the cursor.
The cursor must have previously been initialized.

delete
routines return -1 on error (setting
errno),
0 on success, and 1 if the specified
key
was not in the file.

fd

A pointer to a routine which returns a file descriptor representative
of the underlying database.
A file descriptor referencing the same file will be returned to all
processes which call
dbopen()
with the same
file
name.
This file descriptor may be safely used as an argument to the
fcntl(2)
and
flock(2)
locking functions.
The file descriptor is not necessarily associated with any of the
underlying files used by the access method.
No file descriptor is available for in memory databases.
fd
routines return -1 on error (setting
errno),
and the file descriptor on success.

get

A pointer to a routine which is the interface for keyed retrieval from
the database.
The address and length of the data associated with the specified
key
are returned in the structure referenced by
data.
get
routines return -1 on error (setting
errno),
0 on success, and 1 if the
key
was not in the file.

put

A pointer to a routine to store key/data pairs in the database.

The argument
flag
may be set to one of the following values:

R_CURSOR

Replace the key/data pair referenced by the cursor.
The cursor must have previously been initialized.

R_IAFTER

Append the data immediately after the data referenced by
key,
creating a new key/data pair.
The record number of the appended key/data pair is returned in the
key
structure.
(Applicable only to the
DB_RECNO
access method.)

R_IBEFORE

Insert the data immediately before the data referenced by
key,
creating a new key/data pair.
The record number of the inserted key/data pair is returned in the
key
structure.
(Applicable only to the
DB_RECNO
access method.)

R_NOOVERWRITE

Enter the new key/data pair only if the key does not previously exist.

R_SETCURSOR

Store the key/data pair, setting or initializing the position of the
cursor to reference it.
(Applicable only to the
DB_BTREE
and
DB_RECNO
access methods.)

R_SETCURSOR
is available only for the
DB_BTREE
and
DB_RECNO
access
methods because it implies that the keys have an inherent order
which does not change.

R_IAFTER
and
R_IBEFORE
are available only for the
DB_RECNO
access method because they each imply that the access method is able to
create new keys.
This is only true if the keys are ordered and independent, record numbers
for example.

The default behavior of the
put
routines is to enter the new key/data pair, replacing any previously
existing key.

put
routines return -1 on error (setting
errno),
0 on success, and 1 if the
R_NOOVERWRITEflag
was set and the key already exists in the file.

seq

A pointer to a routine which is the interface for sequential
retrieval from the database.
The address and length of the key are returned in the structure
referenced by
key,
and the address and length of the data are returned in the
structure referenced
by
data.

Sequential key/data pair retrieval may begin at any time, and the
position of the "cursor" is not affected by calls to the
del,
get,
put,
or
sync
routines.
Modifications to the database during a sequential scan will be reflected
in the scan, that is,
records inserted behind the cursor will not be returned
while records inserted in front of the cursor will be returned.

The flag value
must
be set to one of the following values:

R_CURSOR

The data associated with the specified key is returned.
This differs from the
get
routines in that it sets or initializes the cursor to the location of
the key as well.
(Note, for the
DB_BTREE
access method, the returned key is not necessarily an
exact match for the specified key.
The returned key is the smallest key greater than or equal to the specified
key, permitting partial key matches and range searches.)

R_FIRST

The first key/data pair of the database is returned, and the cursor
is set or initialized to reference it.

R_LAST

The last key/data pair of the database is returned, and the cursor
is set or initialized to reference it.
(Applicable only to the
DB_BTREE
and
DB_RECNO
access methods.)

R_NEXT

Retrieve the key/data pair immediately after the cursor.
If the cursor is not yet set, this is the same as the
R_FIRST
flag.

R_PREV

Retrieve the key/data pair immediately before the cursor.
If the cursor is not yet set, this is the same as the
R_LAST
flag.
(Applicable only to the
DB_BTREE
and
DB_RECNO
access methods.)

R_LAST
and
R_PREV
are available only for the
DB_BTREE
and
DB_RECNO
access methods because they each imply that the keys have an inherent
order which does not change.

seq
routines return -1 on error (setting
errno),
0 on success and 1 if there are no key/data pairs less than or greater
than the specified or current key.
If the
DB_RECNO
access method is being used, and if the database file
is a character special file and no complete key/data pairs are currently
available, the
seq
routines return 2.

sync

A pointer to a routine to flush any cached information to disk.
If the database is in memory only, the
sync
routine has no effect and will always succeed.

The flag value may be set to the following value:

R_RECNOSYNC

If the
DB_RECNO
access method is being used, this flag causes
the sync routine to apply to the btree file which underlies the
recno file, not the recno file itself.
(See the
bfname
field of the
recno(3)
manual page for more information.)

sync
routines return -1 on error (setting
errno)
and 0 on success.

Key/Data Pairs

Access to all file types is based on key/data pairs.
Both keys and data are represented by the following data structure:

typedef struct {
void *data;
size_t size;
} DBT;

The elements of the
DBT
structure are defined as follows:

data

A pointer to a byte string.

size

The length of the byte string.

Key and data byte strings may reference strings of essentially unlimited
length although any two of them must fit into available memory at the same
time.
It should be noted that the access methods provide no guarantees about
byte string alignment.

ERRORS

The
dbopen()
routine may fail and set
errno
for any of the errors specified for the library routines
open(2)
and
malloc(3)
or the following:

EFTYPE

A file is incorrectly formatted.

EINVAL

A parameter has been specified (hash function, pad byte etc.) that is
incompatible with the current file specification or which is not
meaningful for the function (for example, use of the cursor without
prior initialization) or there is a mismatch between the version
number of file and the software.

The
close
routines may fail and set
errno
for any of the errors specified for the library routines
close(2),
read(2),
write(2),
free(3),
or
fsync(2).

The
del,
get,
put
and
seq
routines may fail and set
errno
for any of the errors specified for the library routines
read(2),
write(2),
free(3)
or
malloc(3).

The
fd
routines will fail and set
errno
to
ENOENT
for in memory databases.

The
sync
routines may fail and set
errno
for any of the errors specified for the library routine
fsync(2).

BUGS

The typedef
DBT
is a mnemonic for "data base thang", and was used
because no-one could think of a reasonable name that wasn't already used.

The file descriptor interface is a kludge and will be deleted in a
future version of the interface.

None of the access methods provide any form of concurrent access,
locking, or transactions.